The Nd-Fe-B system sintered magnet has a higher maximum energy product (BH)max compared with SmCo.sub.5 system sintered magnets or Sm.sub.2 Co.sub.17 system sintered magnets and is used for various purposes. However, since the Nd-Fe-B system sintered magnet has less thermal stability than Sm-Co system sintered magnets, various trials have been proposed to improve its thermal stability.
Japanese Patent Application Laid-open Print No. 7503/1989 describes a permanent magnet having superior thermal stability which are represented by the following general formulas: EQU R(Fe.sub.1-x-y-z Co.sub.x B.sub.y Ga.sub.z).sub.A
(R is at least one element selected from rare earth elements. 0.ltoreq.x.ltoreq.0.7, 0.02.ltoreq.y.ltoreq.0.3, 0.001.ltoreq.z.ltoreq.0.15 and 4.0.ltoreq.A.ltoreq.7.5), and EQU R(Fe.sub.1-x-y-z Co.sub.x B.sub.y Ga.sub.z M.sub.u).sub.A
(R is at least one element selected from rare earth elements, M is at least one element selected from Nb, W, V, Ta and Mo. 0.ltoreq.x.ltoreq.0.7, 0.02.ltoreq.y.ltoreq.0.3, 0.001.ltoreq.z.ltoreq.0.15, u.ltoreq.0.1 and 4.0.ltoreq.A.ltoreq.7.5).
By adding Ga, this permanent magnet has realized superior thermal stability with an improved coercive force iHc.
Recently, devices using permanent magnets have been further miniaturized and, accordingly, a permanent magnet having both excellent thermal stability and a higher energy product has been desired. The aforementioned permanent magnet is superior in thermal stability but cannot meet the energy product requirement. Permanent magnets are practically required to have coercive forces iHc of 12 KOe or more but permanent magnets providing a coercive force of this level have maximum energy products (BH)max of only 40 MGOe or below.